For preparing quaternary ammonium salts, already known are a process comprising reacting an alkyl halide with a tertiary amine to obtain a quaternary ammonium halide and reacting an acid with the halide, and a process comprising reacting a carbonic acid diester with a tertiary amine to produce a quaternary ammonium salt and subsequently reacting an acid with the salt for decarbonation.
In the case where the tertiary amine is not completely converted to a quaternary compound in either of these processes, the tertiary amine left unreacted will react with the acid in the next step to produce a protonic acid salt of the tertiary amine, permitting the acid salt to remain in the main product of quaternary ammonium salt.
The quaternary ammonium halide or quaternary ammonium salt is likely to produce a tertiary amine when thermally decomposed. The tertiary amine produced provides a tertiary amine protonic acid salt in the process for preparing the quaternary ammonium salt. Since the hydrogen atom of the cation of the tertiary amine protonic acid salt is liable to become released as a proton, the acid salt is unstable to a reducing reaction, so that the acid salt is known to be responsible for the diminution of the voltage resistance and long-term reliability of electric double layer capacitors and electrolytic capacitors (for example, Patent Literature 1).
Accordingly, it is required to reduce the amounts of the tertiary amine and tertiary amine protonic acid salt which are present in quaternary ammonium salts for use as electrolytes in order to obtain highly reliable electric double layer capacitors and electrolytic capacitors.
In reducing the amounts of the tertiary amine and tertiary amine protonic acid salt in quaternary ammonium salts, it is known to recrystallize the salt in an organic solvent (Patent Literature 1, 2).
However, this method is not suitable to practice industrially, for example, because the crystallization leads to a lower yield. Furthermore, it is difficult to effect the recrystallization if the quaternary ammonium salt is a liquid substance at room temperature.
[Patent Literature 1] JP2000-311839A
[Patent Literature 2] JP2004-186246A
An object of the present invention is to provide a process for preparing quaternary ammonium salts by reducing the amounts of a tertiary amine and a tertiary amine protonic acid salt contained as impurities in quaternary ammonium salts.